tests: use raw strings in test-help.t

This avoids SyntaxWarning on Python 3.8 due to invalid \ escapes.

Differential Revision: https://phab.mercurial-scm.org/D5841

tests/test-help.t

@@ -1674,7 +1674,7 b' Test unfound keyword'
 Test omit indicating for help
 
   $ cat > addverboseitems.py <<EOF
-  > '''extension to test omit indicating.
+  > r'''extension to test omit indicating.
   > 
   > This paragraph is never omitted (for extension)
   > 
@@ -1687,7 +1687,7 b' Test omit indicating for help'
   > '''
   > from __future__ import absolute_import
   > from mercurial import commands, help
-  > testtopic = b"""This paragraph is never omitted (for topic).
+  > testtopic = br"""This paragraph is never omitted (for topic).
   > 
   > .. container:: verbose
   >